The fiber laser industry continues to increase laser performance metrics, such as average power, pulse energy and peak power. Increasing the average power of fiber lasers is largely driven by the brightness of laser diode pumps and the ability to couple power into fiber. Pulse energy and peak power on the other hand are respectively driven by the ability to store and extract energy in the fiber while mitigating the nonlinear processes than can have adverse impacts on the temporal and spectral content of the output pulse. Both of these issues can be effectively addressed by using fibers with large core sizes.
The fundamental transverse mode of an optical fiber LP01 has desirable characteristics in terms of beam shape, minimal beam expansion during propagation through free space (often referred to as “diffraction limited”), and optimum focus-ability. Hence, fundamental mode LP01 propagation is often preferred in the fiber laser industry. In reference to FIG. 1A illustrating a cross-sectional view of a conventional fiber 101, as the diameter of core 105 is increased, fiber 101 begins to support the propagation of more than one transverse optical mode. The number of modes supported in a fiber scales with the V-number. The V-number is proportional to the core diameter dcore and core numerical aperture NAcore of the fiber and inversely proportional to the wavelength λ of the light propagating in the fiber:
                    V        =                              π            ⁢                                                  ⁢                          d              core                        ⁢                          NA              core                                λ                                    (        1        )            
In some operating regimes, the number of modes supported by a fiber is given by roughly one half the square of the V-number.
                              M          multimode                ≈                              4            ⁢                          V              2                                            π            2                                              (        2        )            
It can be shown that a fiber with a V-number less than about 2.4 supports the propagation of only the fundamental mode. In Large Mode Area (LMA) optical fibers having a V-number over 2.4, several optical modes may propagate. In LMA fiber, higher-order modes (e.g., LP11 being the next highest mode) may be stripped out of the core of the fiber in favor of LP01 mode propagation. LMA can therefore be distinguished from strongly multimode fiber that supports hundreds of modes, and for which light is not to be guided dominantly in the fundamental mode. One conventional technique to discriminate against the higher-order modes is to coil a LMA fiber at a certain bending radius because higher-order modes typically experience more bending loss than does the fundamental mode. FIG. 1B illustrates a graph of bend loss for an exemplary LMA fiber 101 with a 25 μm diameter core, 0.07NA that supports 5 modes for light with a wavelength around 1 μm.
U.S. Pat. No. 8,711,471, which is incorporated herein by reference in its entirety for all purposes, discloses an optical mode filter employing a three dimensional (3D) mandrel upon which a multimode fiber, such as LMA fiber 101, may be coiled about two orthogonal bending axes to enable stripping of higher-order modes having orthogonal orientations. LMA fiber optical mode filters that offer a similar higher-order mode stripping efficiency, but with a simplified mechanical design may offer many commercial advantages, including a smaller form factor and/or a lower cost.